Wear resistance of high level is required for the piston ring used in a reciprocating internal combustion engine. Therefore, flaky graphite cast-iron material (FC250 or FC300), nodular graphite cast-iron material (FCD700 or the like), and compacted varmicular (CV) graphite cast-iron material proposed in Japanese Unexamined Patent Publication No. Hei 5-86473 or the like have been extensively used heretofore for the piston rings of an internal combustion engine. Also, a cast iron or steel piston-ring, provided with a hard chromium plating layer or a composite dispersion plating layer on the outer peripheral sliding surface for imparting wear resistance, is widely used.
Nevertheless, since the piston ring is caused to slide on the cylinder inner surface at high speed, the piston ring should not only have excellent wear resistance in itself but should also have such property that it does not abrade the cylinder inner-surface, which is the opposed material. Particularly, when the opposed material of the piston ring, i.e., the cylinder liner, is a flaky graphite cast iron, the ferrite precipitation amount of which is increased by lowering the cooling rate at the casting and hence the hardness of which is from HRB approximately 85-95, since the wear resistance of the liner itself is low, a property of the piston ring, that does not abrade the opposed material, is an important factor of the piston ring.
The cast-iron or steel piston ring, which is provided with a hard chromium plating layer or a composite dispersion plating layer on the outer peripheral sliding surface, has an excellent wear resistance in itself but also has a strong abrasive tendency on the flaky graphite cast-iron liner as the opposed material. The above cast-iron or steel piston ring has, therefore, been occasionally used for the 1st ring which is required to have breaking resistance. It was, however, seldom used as the 2nd ring. For the 2nd ring, a piston ring made of flaky graphite cast-iron material or CV graphite cast-iron material has, therefore, been used heretofore, without being provided with a surface-treatment layer. A piston ring consisting of these materials has, however, low wear resistance in itself and low seizure resistance with respect to the opposed material (flaky graphite cast iron). Improvement of these properties is, therefore, desired.